The invention relates to an apparatus for the recording of x-ray images, comprising an x-ray source, an image recording carrier, as well as at least one diaphragm, arranged between the x-ray source and a subject to be examined and exhibiting at least one aperture, for the purpose of generating a first radiation beam, in which the diaphragm is moved relative to the subject and the image recording carrier in such a fashion that the subject is areally irradiated and imaged.
The information content in an x-ray image today is primarily restricted by the secondary radiation which is superimposed on the desired information. For certain subjects this stray radiation can amount to up to 70%. In order to reduce this contrast-reducing effect of stray radiation, already various stray radiation screens (Erich Krestel, Imaging Systems for Medical Diagnostics, Siemens AG Publisher, Berlin and Munich, 1980, pp 190 to 201) are known. In particular, a slit diaphragm mechanism is also known therefrom in which, through a first slit diaphragm between the x-ray source and the subject, and selectively an additional slit diaphragm between the subject and the image recording carrier, delineate from a greater radiation field a narrow strip radiation beam cross section in which the stray radiation is low. In order to image the entire subject the slit diaphragms must be moved over the image field during radiographic recording.
A further contrast-reducing, and hence information-limiting, factor results from the difficulty of adapting the exposure to various parts of the subject if the latter exhibit great variations in density and hence in x-ray absorption.
Too soft x-radiation is entirely absorbed in the denser subject regions, so that the latter can virtually not be imaged. If one selects so hard an x-radiation that all subject regions are imaged, this impairs the contrast, specifically in the regions of lower density. A certain remedy is here provided by additional absorption layers, so-called Dogers, which are arranged over the subject regions of low density. However, through these "Dogers", also the danger of an image falsification exists, since they are jointly imaged, as a consequence of which an additional image is superimposed on the actual subject image. Moreover, they cause stray radiation.
The most reliable way for high-contrast imaging of variously dense subject parts previously consisted in making several radiographs with varying exposure in which respectively only partial regions were optimally exposed and hence imaged in high-contrast fashion. The varying exposure was adjusted through alteration of the exposure time.